Hydrocarbon conversion with platinum second metal catalysts in proportions which form ordered alloys

ABSTRACT

A HYDROCARBON CONVERSION CATALYST OF 0.01-10% WT. METAL ON A REFRACTORY SUPPORT CONTAINS PLATINUM AND A SECOND METAL WHICH FORMS A SOLID SOLUTION WITH PT, THE ATOMIC AMOUNTS OF EACH BEING EQUIVALENT TO AMOUNTS FORMING ORDERED ALLOY STRUCTURES. THE SECOND METAL MAY BE CO, NI, FE, CU, SN, PD (PARTICULARLY THESE SIX), IR, RH, AG, AU, BI, HG, SB, PD OR CD AND PREFERABLY THERE IS AT LEAST 45 ATOMIC PERCENT PT. THE CATALYSTS MAY BE PREPARED BY KNOWN IMPREGNATION OR ION-EXCHANGE TECHNIQUES AND ARE PREFERABLY REDUCED BEFORE USE AT 250-600*C. THE PREFERRED USE IS DEHYDROCYCLISATION OR DEHYDROGENATION OF C3-C25 HYDROCARBONS, PARTICULARLY THE CATALYTIC REFORMING OF 15-204*C. BOILING RANGE PETROLEUM FRACTIONS.   D R A W I N G

Sept. 18, 1973 E. E DAVIES ET AL 3,759,823 HYDROCARBON CONVERSION WITH ILA'l'lNUM-SECOND-METAL. CATALYSTS 1N PROPORTIONS wmcn FORM ORDERED ALLOYS Filed March 50, 1971 2 Sheets-Shed 2 T Fig. 2 9 A AP//V//4/ 05 -P/F6//1/ 0 E MPH/H205 *3 -60 Q 0-2- /A-\ RATE OF FORMATION OF BENZENE M/L I I ATOM/C PE R cuvr Pt, I

5 Pt Fe Pu e? ArraxM FVS United States Patent US. Cl. 208-138 5 Claims ABSTRACT OF THE DISCLOSURE A hydrocarbon conversion catalyst of 0.01-% wt. metal on a refractory support contains platinum and a second metal which forms a solid solution with Pt, the atomic amounts of each being equivalent to amounts forming ordered alloy structures. The second metal may be Co, Ni, Fe, Cu, Sn, Pd (particularly these six), Ir, Rh, Ag, Au, Bi, Hg, Sb, Pd or Cd and preferably there is at least 45 atomic percent Pt. The catalysts may be prepared by known impregnation or ion-exchange techniques and are preferably reduced before use at 250-600 C.

The preferred use is dehydrocyclisation or dehydrogenation of C -C hydrocarbons, particularly the catalytic reforming of -204 C. boiling range petroleum fractions.

3,759,823 Patented Sept. 18, 1973 ice According to the present invention, therefore, a catalyst suitable for the conversion of hydrocarbons comprising from 0.01 to 10% wt. of metal on a refractory support is characterised in that it contains platinum and a second metal which forms asolid solution with platinum, the atomic amounts of the platinum and the second metal being substantially equivalent to amounts which form ordered alloy structures.

The metallurgy of platinum alloys is well established and metals known to form solid solutions with platinum include cobalt, nickel, iron, copper, tin, palladium, iridium, rhodium, silver, gold, bismuth, mercury, antimony, lead, and cadmium. In accordance with established metallurgical principles the existence of ordered structures in any bi-metallic system can be readily determined, and many of them are already known. These ordered structures have atomic proportions of the two metals in a simple ratio. The ratios may vary with diiferent metals but for any two metals there will be relatively few ordered structures, usually 5 at the most. The most common are AB i.e. atomic percent A, 50 atomic percent B A B i.e. 75 atomic percent A, 25 atomic percent B AqB i.e. 87 /2 atomic percent A, 12 /2 atomic percent B Others which may exist in certain systems are A B and A B but, in any system, they can, as indicated above, be detected by standard techniques.

With the second metals listed above the following proportions of platinum and the second metal are known to give ordered structures Ptlsb This invention relates to platinum-containing catalysts and their use for the catalytic conversion of hydrocarbons.

Catalysts of platinum on a refractory support are well known. The platinum, which may be present in an amount of 0.1 to 5% wt., has good activity for hydrogenation or dehydrogenation and dehydrocyclisation depending on the process conditions. By a suitable choice of support, additional functions of e.g. isomerisation and cracking can be given to the catalyst.

Since platinum is an expensive metal, methods of reducing the platinum content of such catalysts without affecting their performance are potentially useful, and considerable interest has been shown recently in the development of bi-metallic catalysts. It has now been found that both the nature of the second metal and the proportion of this metal in relation to the platinum are important. In particular, it has been found that graphs of catalyst activity against the amounts of metals show distinct peaks corresponding with particular compositions.

The ordered structures Pt Sn and PtCo, have not been previously reported bu their existence may be presumed from results described hereafter. There is published evidence for the existence of ordered platinum-palladium structures at about PtqPd and PtPd but their exact composition is less clear. The possibility of ordered structures from platinum-iridium and platinum-rhodium alloys exists but the published evidence is not conclusive.

The term substantially equivalent to amounts which form ordered alloy structures means i 5 atomic percent for each metal.

Preferably the platinum is substantially equal to or greater than the amount of the other metal. The platinum is preferably thus at least 45 atomic percent. The preferred second metals are cobalt, nickel, iron, copper, tin and palladium.

According to another aspect of the invention method of preparing a catalyst suitable for the conversion of hydrocarbons comprising from 0.01 to 10% wt. of metal on a refractory support comprises contacting the support with a solution containing platinum ions and a solution containing ions of a second metal which forms a solid solution with platinum, the concentration of ions in the solutions and the conditions of contacting being such that the amounts of platinum and the second metal on the catalyst are substantially equivalent to amounts which form ordered alloy srtuctures.

The support may be contacted simultaneously with solutions containing ions of platinum and the second metal or sequentially in either order. Preferably the contacting is carried out sequentially with the platinum containing solution being used first.

The platinum and the second metal may be added by ion-exchange, this being a known term indicating that the ions combine chemically with active sites on the surface of the support. Particularly in the case of supports having few or weak active sites, the support should be washed exhaustively with water while the ions are still in a water-soluble state until the wash water is free of metal ions. This washing ensures that the only metal remaining on the support is in ion-exchanged form. The platinum and the second metal ions can be added in either cation or anion form, the former being preferred since the most commonly used supports are cation-exchangers.

The platinum and the second metal may also be added by impregnation in conventional manner and this is, in fact, preferred.

The term metal ions includes complex ions where the metal is attached to a ligand, particularly water, ammonium, or halogen. Such complex ions are the usual form of ions for many metals in aqueous or ammoniacal solution. Thus the normal platinum containing solutions used in catalyst preparations are tetrammine platinous chloride giving tetrammine platinous cations and chloroplatinic acid giving hexachloroplatinate anions. The second metal solution may contain any suitable salt, e.g. chloride, nitrate, or acetate and may again be an ammoniacal solution.

The conditions of the contacting with platinum and the second metal ionsthe temperature, time and concentration of ion in solutionwill depend on the ease of uptake and the desired uptake, and may be readily determined by experiment if necessary. Suitable conditions have been found to include temperatures of -110" C., times of 1 to 72 hours and solution concentrations of 0.001 to 2 molar.

When preparing ion-exchanged catalysts, the washing to remove uncombined ions desirably uses de-ionised water and, as indicated above, is continued until the wash water is free of ions of platinum and the second metal. The temperature may also be 10 to 110 C., and the time 1 to 72 hours using, preferably, 2 to 100 ml. of water/ml. of catalyst.

The refractory support is preferably an inorganic oxide of an element of Groups II, III and IV of the Periodic Table, a mixture of two or more such oxides or a compound containing one or more of such oxides in its empirical formula. Preferred individual oxides are silica or alumina, preferred mixed oxides are silica-alumina, silica-magnesia, or boria-alumina, and preferred minerals are alumina-silicates e.g. zeolites.

The catalysts may also contain from 0.1 to 8% wt. of halogen, particularly chlorine.

After the addition of the metals the catalyst may be dried e.g. at 50-110 C. for 1 to 24 hours, and calcined at 250 to 600 C. for 1 to 24 hours. Desirably the catalysts are also reduced before use by heating them in a reducing atmosphere at 200 to 600 C. for 1 to 24 hours. The reducing atmosphere is preferably a flowing stream of hydrogen. The reduction of the dual-metal catalyst may be more difficult than that of catalysts containing only platinum and care should be exercised to ensure reduction. The extent of reduction can be monitored by hydrogen uptake from a closed system in which hydrogen is circulated over the catalyst.

The present invention includes a process for the catalytic conversion of hydrocarbons comprising contacting the hydrocarbons under conversion conditions with a catalyst containing platinum and a second metal on a refractory support having a composition as previously described.

Conversion conditions can vary widely depending on the feedstock and reaction but they are normally within the ranges Temperature C 0-600 Pressure p.s.i.g 0-3000 Space velocity v./v./hr 0.1-20' Hydrogemhydrocarbon mole ratio 0-20z1 The preferred hydrocarbon feedstocks may be derived from any convenient source e.g. from petroleum and their precise nature will depend on the reaction required.

For dehydrogenation and dehydrocyclisation the preferred hydrocarbons are parafiins and/ or olefins and/or naphthenes, particularly those having from 3 to 25 carbon atoms. The preferred dehydrogenation or dehydro cyclisation reactions may be operated under the following ranges of conditions:

Temperature C 300-600 Pressure p.s.i.g 01000 Space velocity v./v./hr 0.1-10

Thus the catalysts of the present invention may be used for the catalytic reforming of hydrocarbons boiling in the gasoline range 15 to 204 C.), particularly petroleum fractions, to increase the aromatic content and/ or octane number. The preferred support for such use is alumina, possibly containing from 0.1 to 8% wt. of halogen, such a support having the moderate isomerisation and cracking activity usually considered desirable in catalytic reforming.

For hydrogenation reactions the feedstocks may be unsaturated hydrocarbons e.g. acetylenes, olefins, or aromatics, particularly those having'from 2 to 20 carbon atoms. The process conditions may be chosen from:

Hydrogenzhydrocarbon mole ratio Temperature C... 0300 Pressure p.s.i.g 0-2000 Space velocity v./v./hr 0.1-20 Hydrogenzhydrocarbon mole ratio 0.001 to 20:1

For hydrogenation reactions the catalyst support is preferably relatively inert and may be for example silica, sepiolite or low-acidity alumina.

The invention is illustrated by the following examples.

EXAMPLE 1 A series of platinum-copper catalysts was prepared. The support used was silica of 30-60 B.S.S. mesh obtained from Hopkins and Williams Ltd. having a surface area of 170 m. g. It was freed from iron impurities by washing with normal HCl and distilled water. A catalyst containing 1.8% wt. platinum was prepared by contacting g. silica with 9.22 ml. of a molar solution of tetrammine platinous chloride at 25 C. for 2 h. The silica was then washed with 50 ml. aliquots of deionised water at 25 C. The final wash water was free of platinum ions. The catalyst was dried at C. for 24 hours.

A series of platinum and copper catalysts was prepared by progressively reducing the concentration of tetrammine platinous chloride in the solution and contacting the Pt- SiO with solutions of cupric chloride in 0.880 S.G. aqueous ammonia of progressively increasing concentration. The relative concentrations were adjusted so that each catalyst had the same gram atom metal content as the 1.8% wt. Pt. on silica but varying atomic ratios of Pt and Cu, 90:10, 80:20, 70:30 and so on. In the final catalyst the contacting with platinum was omitted altogether giving a catalyst of 0.59% wt. copper on silica. Washing and drying after each contacting with copper solution was carried out as for the contacting with platitested for dehydrocyclisation activity using the n-hexane num solution. feedstock and process conditions of Example 1 (ie Each catalyst was dried at 110 C. for 24 hours and 500 C., atmospheric pressure, 4000 v./v./hr. of feedstock reduced in 4000 v./v./hr. of hydrogen at 500 C. for having a 10:1 H m-hexane mole ratio).

3 hours. Examination by electron spin resonance and The results obtained are shown in Table 2 below,

measurement of the hydrogen uptake during reduction expressed as conversion rates in millimoles, minr confirmed that the metal had been reduced. Each catalyst gram- TABLE 2.PLATIN UM-C OPPE R-AL UMINA Selectivity Selectivity Selectivity Selectivity Total confor to al Total Conversion for benzene, Conversion for toluene, Conversion for xylenes, version to aromatics, Atomic percent Pt conversion to benzene percent to toluene percent to xylenes percent aromatics percent 0. 25 0. 11 44. 2 0.02 7. 3 0. 13 51. 5 0. 33 0. 16 49. 5 0. 03 7. 5 0. 02 4. 8 0. 21 61. 8 0. 25 0. 13 51. 6 0.02 8. 9 0.03 10 0. 18 70. 5 0. 27 0. 20 73. 4 0. 03 9. 3 0. 03 12. 3 0. 26 95 0.15 0.11 65.0 0. 01 6. 4 0. 01 0. 3 0.13 78. 2 0. 30 0. 15 51 0. 02 8. 5 0. 04 12. 9 0. 21 72. 5 0. 21 0. 07 33 0. 01 9. 5 o. 02 9. 5 0. 11 52 0. 17 0. 07 41 0. 01 5. 9 0. 02 11. s 0. 10 58. 7 0. 17 0. 06 35 0. 01 5. 9 0. 03 11. 8 0. 09 52. 7 0. 12 0. 05 42 0. 01 8. 3 0. 03 0. 09 75. 3 0. 04 Trace Trace Trace 0. 22 68 was used to dehydrogenate and dehydrocyclise n-hexane EXAMPLE 3 I o I n n a I i a g i g f g f fl The figf Alumina, obtained by calclnation of an alumina hydrate y g g an t 2655? exam mo 6 1 precursor in which the trihydrates predominated, having was e l h j bl 1 b 1 d 25 a surface area of 242 m. /g., a pore volume of 0.37 ml./g., The i ts S in? a Ow ff gg as and a particle size of 60 to 80 BSS mesh was impregnated converslon rates m1 65, gram X with sufficient chloroplatinic acid to give a catalyst con- TABLE 1 taining 0.6% wt. platinum. A series of platinum and cobalt catalysts was also prepared by progressively reducing the Atomic percent concentration of chlor-platinic acid in the solution and 39 5 ggg g impregnating the Pt-Al O with solutions of cobalt chloride of progressively increasing concentration. The relative amounts were adjusted so that each catalyst had the millimoles min.- g.- Xi0 18.0 5.1 13% same total gram atom metal content as the 0.6% wt. Pt on alumina but varying atomic ratios of Pt and Co 90:10, 80:20, 70:30 and so on. A catalyst with 100% Co was also 515 112 p p g- Each catalyst was dried at 110 C. overnight, reduced in 4000 v./v./hr. of hydrogen at 500 C. for 3 hours, and 40 tested for dehydrocyclisation activity using n-hexane as The conversion products other than benzene were feedstock and process conditions of 500 C., atmospheric i l hexenes d oke, pressure, and 4000 v./v./hr. of feedstock having a 10:1

H m-hexane mole ratio. EXAMPLE 2 The results obtained are shown in Table 3 below, Alumina, obtained by 63101113111011 Of an alumina hydrate expressed 35 conversion rates in millimoles, mi

precursor in which the trihydrates predominated, having a gram- TABLE 3.PLATINUMCOBALT-ALUMINA Rate of formation, millimoles m.ln.- gmr Selectivity Total Atomic conversion, percent Total miilimoles Pt Benzene Toluene Xylenes Benzene Toluene Xylenes aromatics mingm.-

.13 015 .018 46 5 5.2 Y 5.3 58.0 0.23 0.15 .014 .019 58 5.5 7.4 70.9 0. 25 0.18 017 .024 64 5 5. 2 8.7 79. 4 0. 23 0. 15 015 025 5. s 10 73. s 0. 2e 0. 14 014 020 51 5. 9 8.6 75. 5 0. 23 0. 17 015 027 08 (s. 3 10. 8 s5. 1 0. 25 0. 09 013 .017 39 5. 5 7. 5 43. 1 0. 23 0. 05 00s 028 27 s 4. 5 15 47. 3 0.18 o. 05 013 03s 35 7. s 22. 4 05. 2 0. 17 003 009 10 30 40 0. 03 .004

surface area of 242 m. /g., a pore volume of 0.37 ml./g., The results for benzene formation from the n-hexane and a particle size of to 80 BSS mesh was impregnated feedstock (i.e. dehydrocyclisation activity) obtained in with sufficient chloroplatinic acid to give a catalyst con- Examples 1, 2, and 3 are expressed graphically in the taining 0.6 wt. platinum. A series of platinum and copper accompanying FIG. 1. The graph clearly shows the large catalysts was also prepared in a manner similar to Exam- 65 variations in activities depending on the Pt-Cu and Pt-Co pie 1 by progressively reducing the amount of platinum atomic ratios and the pronounced peaks corresponding to added and impregnating the Pt-Al O- with cupric chloordered structures. Those ordered structures with high ride solution to give progressively increasing copper conplatinum contents gave significantly higher activities than tents. The relative amounts were adjusted so that each the catalyst with 100 atomic percent Cu. catalyst had the same total gram atom metal content as the The peak at Pt Cu is to be compared with the absence of 0.6% wt. Pt on alumina but varying atomic ratios of Pt any peak with Pt-Co, there being no corresponding and Co, :10, 80:20, 70:30 and so on. A catalyst with ordered structure in the Pt-Co system. No measurements Cu was also prepared. were made at 75% Pt 25% Cu or 75 Pt 25 Co (Pt Cu Each catalyst was dried at C. overnight, reduced in and Pt Co respectively) so the peaks are slightly displaced. 4000 v./v./hr. of hydrogen at 500 C. for 3 hours, and 75 Although the Pt-Cu-Si0 catalysts were of low activity, the

7 8 presence of peaks can still be seen. The catalysts contain- The platinum-tin-alumina catalysts showed the following 40 atomic percent or less of platinum had relatively ing results for rate of benzene formation low activities but the curves still show a non-linear elfect Rate of nz a i n in this region, including a definite peak in the Pt-Co curve. be ens form to Percent Pt: millimoles minr gram- EXAMPLE 4 5 100 0.13

A series of platinum-nickel-alumina and platinum-iron- 90 '296 alumina catalysts were prepared using the same technique 80 0'219 as in Example 3 but using nickel chloride (NiCl or iron 70 0'267 chloride (FeCl in place of cobalt chloride. The catalysts 1Q were tested for dehydrocyclisation activity again using the feedstock and process conditions of Example 3. The results Again with these catalysts, variations throughout the are shown in Tables 4 and 5 below. range are seen. The platinum-palladium-alumina catalysts TABLE 4.PLATIN UM-NICKEL-ALUMINA Rate of formation, millimoles min: gram- Selectivity Atomic Total Total percent Pt conversion Benzene Toluene Xylenes Benzene Toluene Xylenes aromatics TABLE 5.PLATINUM-IRONALUMINA Rate of formation, millimoles minr gram- Selectivity Total Benzene Total Percent Pt conversion formation Xylenes Toluene Benzene Xylenes Toluene aromatics The results for benzene formation obtained in Examof Table 6 show maxima at 90 PtzlOPd (corresponding ple 4 are expressed graphically in the accompanying to the probable ordered alloy structure PtqPd) and FIG. 2. Also included in FIG. 2 are results with a series of 50 Pt:50 Pd (corresponding to the probable ordered alloy platinum-alumina catalysts of decreasing platinum constructure PtPd).

tent. As with FIG. 1 peaks corresponding to ordered The platinum-tin-alumina results are particularly interstructures can be clearly seen, these peaks showing in most esting not only in the peaks at 90:10 (Pt sn) :30 instances, a higher activity than a catalyst of 100 atomic (Pt Snx) and 50:50 (PtSn) but also in the marked percent platinum. The straight-line graph for the Pt-Al O increase of activity conferred by the tin as compared with catalysts of decreasing platinum contents provides a base 50 the 100% Pt catalyst. line for comparison and shows that the peaks are not due Comparing the best result with each metal in the to any platinum dilution effect. Examples 2-5 with 100 atomic percent Pt the relative EXAMPLE 5 improvement by the incorporation of a second metal in a 5'? proportion equivalent to an ordered alloy structure is A series of platinum-palladium-alumina catalysts and a 0 series of platinum-tin-alumina catalysts (over the range 100 atomic percent Pt 100 100-50 atomic percent Pt) were prepared by the Same 70% Pt 30% Cu (Pt Cu) 15S technique as in Example 3 but using PdCl and SnCl in Pt 20% Co (Pt Co) 140 place of cobalt chloride. The catalysts were tested for 50% Pt 50% Ni (PtNi) 155 dehydrocyclisation activity again using the feedstock and 60 80% Pt 20% Fe (Pt Fe) 145 process conditions of Example 3. The results for the Pt 10% Pd (Pt Pd) Pt-Pd-AI O catalysts are shown in Table 6 below. 90% Pt 10% Sn (Pt Sn) 225 TABLE 6.PLATINUM-PALLADIUM-ALUMINA Rate of formation, millimole min: gram- Selectivity Total Total Percent Pt conversion Benzene Xylenes Toluene Benzene Xylenes Toluene aromatics 0. 26 0.15 .006 007 60. 5 0.27 0.14 .004 .004 55.5 0. 27 0. 11 001 .003 42. 3 0. 22 0. 0523 Trace 001 40. a 0. 2e 0. 12 Trace 004 48 0. 2e 0. 11 Trace .004 44 0. 20 0. 002 .009 004 52. 5 0.19 0 080 Trace .003 43.0 0. 08 038 011 Trace 43. 3 0. 07 014 Trace Trace 20 9 EXAMPLE 6' Samples of the platinum-copper-alumina catalysts of Example 2 having the atomic proportions 70:30, 60:40, and 50:50 were tested for an extended period at elevated pressure under the following conditions The products were analysed by gas liquid chromatography and the conversion of the n-heptane to aromatics was as follows:

Conversion to aromatres, mol percent Initial Final 70% Pt, 30% Cu 50 40 60% Pt, 40% Cu 1.- 80 20 50% Pt,50% Cu 45 35 These results confirm the previous findings that catalysts with metal contents corresponding to ordered alloy structures (70% Pt 30% Cu=Pt Cu and 50% Pt 50% Cu=PtCu) are significantly more active than a catalyst of 60% Pt 40% Cu corresponding to no ordered alloy structure. The results confirm that this finding applies at elevated pressure and that it is maintained for extended periods.

We claim:

1. A process for the catalytic conversion of hydrocarbons comprising contacting the hydrocarbons at a temperature of to 600 C., a pressure of 0 to 3000 p.s.i.g., a space velocity of 0.1 to 20 v./v./hr. and a hydrogenzhydrocarbon mole ratio of 0 to 20:1 with a catalyst comprisin from 0.01 to wt. of metal on a refractory support, said catalyst containing platinum and a second metal which forms a solid solution with platinum, said second metal being selected from the group consisting of cobalt, nickel, iron, copper, tin, palladium, iridium, rhodium, silver, gold, bismuth, mercury, antimony, lead and cadmium, the atomic amount of platinum in said solid solution being at least atomic percent and being also within :5 atomic percent of the atomic amount of platinum required to form an ordered alloy structure with said second metal, and the atomic amount of said second metal in said solid solution being within :5 atomic percent of the atomic amount of said second metal required to form said ordered alloy structure with the platinum.

2. A process as claimed in claim 1 which is the dehydrogenation or dehydrocyclisation of hydrocarbons having from 3 to 25 carbon atoms at a temperature of 300 to 600 C. a pressure of 0 to 1000 p.s.i.g., a space velocity of 0.1 to 10 v./v./hr. and a hydrogen: hydrocarbon mole ratio of 0 to 20:1.

3. A process as claimed in claim 2 which is the catalytic reforming of a petroleum fraction boiling in the range 15 to 204 C.

4. A process as claimed in claim 1 wherein the second metal is selected from the group consisting of cobalt, nickel, iron, copper, tin and palladium and wherein the platinum and the selected second metal are in atomic amounts which form any one of the following ordered alloy structures: Pt Co, PtCo; PtNi; Pt Fe, PtFe; Pt Cu, Pt Cu, PtCu; Pt Sn, Pt Sn, PtSn; P qPd, PtPd.

5. A process as claimed in claim 1 wherein the support is alumina.

References Cited UNITED STATES PATENTS 2,911,357 11/1959 Myers et a1 208-138 2,906,700 9/1959 Stine et al 208 138 3,562,346 2/1971 Smirnov et al 208138 3,617,518 11/1971 Sinfelt 208138 OTHER REFERENCES Elliott: Constitution of Binary Alloys, (1965, p. 17, Pub. McGraW-Hill, N.Y.

HERBERT LEVINE, Primary Examiner US. Cl. X.R.

208l43; 252466 B, 466 PT; 260-6735 ro-wso Patent No. 3,759,823 7 Dated September 18, 1973 Inventor) Evan Ellis Davies, John Stanley Elkins, Robert Chalmers PTtkethly It; is certified that error appears in the above-identified patent and thateaid Letters Patent are hereby corrected as shown helm" Column 2, line 48, change "bu" to but 4 Column 5, line 64, change "0.6" to 0.6%

Column 5, line 72, change "C0" to u Column 7, Table 5, second column from right, under "3.5"

i 1 insert 2 I Signed and sealed this 12th day of March 1974.

SEAL) EDWARD M.FLETCHER,JR c. MARSHALL DANN Attesting Officer Commissioner of Patents 

